This invention relates to magnetic tags and, more particularly but not exclusively, is concerned with tags which can be encoded with data.
In previous patent applications, notably in WO96/31790 and WO97/04338, we have described remote identification tags which may be interrogated using alternating and/or static magnetic fields. In response to interrogation the tags emit magnetic signals which may be detected in suitable receiving equipment. Such tags have many applications in areas such as article identification, security and access control.
Certain of the tags described in WO96/31790 contain elements of high permeability, low coercivity, "soft" magnetic alloy in the form of a strip, wire or thin film, overlaid with a medium coercivity magnetic layer. The magnetic signal or "signature" generated by the high permeability element in response to interrogation is determined by the magnetisation pattern stored in the medium coercivity "coding" layer, and the characteristics of the high permeability element.
In practical implementations tag coding can for example be carried out during tag manufacture by contact- or near contact-recording the appropriate magnetic pattern onto a continuous coding layer using an array of permanent magnets. This method is particularly suitable for producing many tags with the same code.
Another method, which is suitable for manufacturing tags whose code will never require change, is to place suitably-magnetised pieces of coding material at appropriate positions on the high permeability material.
Yet another method is to place un-magnetised pieces of medium coercivity material in an appropriate pattern using e.g. a thermal transfer process, and then to bulk-magnetise the entire structure.
A very convenient arrangement uses a magnetic recording head of the type well-known for recording information onto magnetic tape. With this arrangement, tags manufactured with a uniform un-magnetised coding layer can have patterns individually coded at the point-of-issue. This is extremely useful for applications such as airline baggage tagging, where individual tag details (passenger name etc.) are not known in advance of issue.
WO97/04338 discloses a magnetic marker or tag, which comprises (a) a first magnetic material characterised by high permeability, low coercivity and a non-linear B-H characteristic; and (b) a second magnetic material which is capable of being permanently magnetised, characterised in that said second magnetic material is magnetised with a non-uniform field pattern.
U.S. Pat. No. 5,643,686 (D1) discloses the use of a fixed permanent magnetic pattern for identifying a magnetic recording medium. With this system, a high degree of security against forgery is obtained. The magnetic medium comprises a non-magnetic substrate which supports at least two magnetic layers one of which contains dispersed magnetic particles having a coercivity of 4000 oersted or less and has a non-rewritable fixed signal recorded into the layers. This is achieved by magnetically orienting the dispersed particles during manufacture of the layered material while the particle dispersion is still wet. The orienting magnetic field is active until the layer has become solid, whereupon the dispersed particles have acquired the desired non-erasable magnetic pattern.
EP 0353040-A (D2) discloses the application of the so-called "Watermark"-type magnetic material to a soft magnetic EAS (electronic article surveillance) tag. The "Watermark" material comprises magnetic particles orientated into alternating directions differing in phase by 90 degrees within adjacent regions. In its initial state, the "Watermark" material, although structured during manufacture, is unmagnetised. It thus has no effect upon the soft magnetic material in the EAS tag, and the tag can be detected by conventional a.c. interrogation (e.g. as commonly used at detection gates in stores). To prevent detection of the tag, it is wiped, at the point of sale, with a permanent magnet. This causes alternate regions of the "Watermark" layer to become magnetised; these regions then exert a uniform magnetic bias which magnetically clamps adjacent regions of the soft magnetic material, thereby magnetically dividing the material into regions too small to generate an alarm signal when interrogated by an a.c. detection system. There is no magnetic pattern within the magnetised regions--just a uniform magnetisation.